1. Field of the Invention
The present invention relates to an information record medium such as an optical disk of a high recording density type, which is capable of recording information such as video information, audio information and the like at a high density, and which is represented by a DVD (Digital Video or Versatile Disk). The present invention also relates to a recording apparatus for recording the information onto the information record medium, and a reproducing apparatus for reproducing the information from the information record medium.
2. Description of the Related Art
Recently, as a data compressing technique to effectively utilize the memory capacity of an information record disk such as an optical disk for recording video information and the like, a variable rate system such as the MPEG 2 (Moving Picture Experts Group 2) method and the like is proposed. In general, in case of reducing the information amount by use of the compressing technique, the information amount after compression is not constant due to the property of the original signal and the like. This variable rate system is not a system which compress the information at a constant rate in case of recording the information after compression onto the record medium, but is a system which records the information after compression onto the record medium while compressing the information by a data or information compression rate, which is set as a rate appropriate and variable from scene to scene, by use of mutual relationships between the units of data or information successively recorded and reproduced, such as the units of video data related to one scene and the like.
In order to reproduce the data or information, which is recorded by this type of variable rate system and which information amount required for the reproduction (per unit time) is changed, from the record medium, there may be proposed two methods as following.
As a first method, there is a method of changing a linear velocity at a read position of an optical pickup for reading out the information in the reproducing apparatus with respect to a record portion of the information record disk (hereinbelow, it is referred to as a "reading linear velocity") at the time of reproducing the information, in accordance with the information amount after compression of the information record disk. The time change of the information amount after compression, which is read out per unit time at the time of reproducing the record information (hereinbelow, it is referred to as an "information rate"), and the time change of the rotation number of the disk i.e. The reading linear velocity at the time of reproduction according to this first method are shown in FIG. 17. As shown in FIG. 17, according to this first method, the data reading rate i.e. The reading linear velocity (or the rotation number) is changed in accordance with the information rate from time to time so as to always obtain the information rate required for the reproduction by the reproducing apparatus. In FIG. 17, the difference between the information rate, which is actually reproduced in accordance with the reading linear velocity (indicated by a dashed line), and the information rate of the record information after compression (indicated by a solid line), is corresponding to a loss at the time of reproduction due to various factors such as a reproduction of additional information and the like. As a result, since the data amount obtained through the optical pickup and the data amount required for the reproduction are almost coincident to each other, it is possible to reduce the size (i.e. The memory capacity) of a track buffer of the reproducing apparatus. Further, it is possible to restrain the consumption of the electric power, since the rotation number of the spindle motor for rotating the disk can be reduced to be a level of minimum requirement. Furthermore, a special control, such as a switching control for switching the stand by or waiting condition and the reproducing condition of the optical pickup and the like, is not necessary.
As a second method, there is a method of setting the data reading rate i.e. The reading linear velocity to match the upper limit of the information rate required for the reproduction (i.e., so as to be able to reproduce even in case of the upper limit) as shown in FIG. 18, and rotating the spindle motor at a speed to make this data reading rate constant. As a result, the frequency of the read out clock is made constant, so that it is enough to prepare just one PLL (Phase Lock Loop) circuit of normal type for the demodulation of the data.
However, according to the aforementioned first method, since the frequency of the read out clock is always changed, a special PLL circuit is required for the demodulation, the control of such a special circuit becomes complicated, and the rotation control of the spindle motor becomes also complicated, resulting in that this first method is not practical.
On the other hand, according to the aforementioned second method, since the data amount used for the demodulation is greatly changed, it is necessary for the optical pickup to repeat the reproducing condition, in which the optical pickup successively read out the information, and the stand by condition, in which the optical pickup is standing by on a same track, so that the control of the optical pickup becomes complicated. Further, since the difference between the data amount read out from the optical pickup and the data required for the demodulation is so large that it is necessary to install the reproducing apparatus with a track buffer of large size (i.e. large memory capacity). Furthermore, since the motor is always rotating in accordance with the highest speed, the power consumption is also increased. Therefore, especially in case of a portable player, the life of the battery is shortened, resulting in that the convenience in usage is certainly degraded.
On the other hand, a proposal and a development are being made as for the DVD, as an optical disk in which the memory capacity is improved by about ten times without changing the size of the optical disk itself as compared with the aforementioned conventional CD. As for the DVD, it is expected that the information can be compressed and recorded by the aforementioned variable rate system. However there is no proposal or development made as for a DVD technique to solve the problems owned by the aforementioned first and second methods. Further, in the technical art of the DVD, the actuality is such that a person having an ordinary skill in this art does not even recognize the problem or subject itself which happens at the time of applying the compression technique, such as the variable rate system, with respect to the DVD, which has the large memory capacity.